Flame structure and stabilization characteristics of autoignited laminar/turbulent lifted jet flames in heated coflow air

Department of Mechanical EngineeringNonpremixed flames have received great attention during the past decades because they have been utilized in many practical combustors such as diesel engines, gas turbines, and boilers. Since the flame is essentially lifted from the burner nozzle, it is inevitably vulnerable to blowout by aerodynamic strain rate induced by a high speed fuel jet. As such, the stabilization of lifted jet flames is one of the key design parameters for development of such combustors and its fundamental stabilization mechanisms have long been a research topic in the combustion community. However, the stabilization mechanisms of laminar/turbulent lifted jet flames are quite complicated such that it still remains challenging to develop a predictive model for accurate description of their characteristics. Therefore, the scope of the work is to investigate the flame structure, liftoff height, and stabilization mechanisms of laminar/turbulent lifted jet flames, especially under autoignitive condition. Under relatively low temperature and high hydrogen ratio conditions of autoignited laminar methane/hydrogen jet flames, an unusual decreasing liftoff height, HL, behavior with increasing fuel jet velocity, U0 is numerically observed, qualitatively similar to those of previous experimental observations. From additional simulations with modified hydrogen mass diffusivity, it is substantiated that the unusual decreasing HL behavior is primarily attributed to the high diffusive nature of hydrogen molecules. For autoignited laminar lifted dimethyl ether (DME) jet flames in heated coflow air, a 'U'-shaped HL behavior under MILD combustion condition is observed from the simulations, which is qualitatively consistent with previous experimental results. From additional numerical simulations with modified mass diffusivity of hydrogen, it is verified that the decreasing HL trend of the lifted flames under relatively-low U0 conditions is mainly attributed to the fast diffusion of hydrogen generated from the DME pyrolysis. The effects of DME pyrolysis on the characteristics of the autoignited laminar DME jet flames are further investigated by varying the fuel tube length. The characteristics of the flame structure and stabilization of autoignited laminar lifted $n$-heptane jet flames in heated coflow air are also investigated by performing 2-D numerical simulations. The present simulations can reproduce a distinct transition of a lifted jet flame from a tribrachial edge flame mode to MILD combustion mode observed from a previous experimental study, featuring a significant variation in the liftoff height with U0. It is found that a lifted flame with MILD combustion mode can exist further downstream of the stoichiometric mixture fraction isoline due to autoignition occurring upstream of the flamebase. Based on the flame structures and stabilization mechanisms of the lifted flames, a flame regime diagram is constructed in the normalized U0 and Damkohler number space. Three-dimensional direct numerical simulations of turbulent lifted hydrogen jet flames in heated coflows are performed with a detailed H2/air chemical mechanism to understand their ignition dynamics and stabilization mechanisms. Turbulent lifted jet flames with four different coflow temperatures, Tc, between 750 K and 1100 K are investigated by examining the instantaneous/time-averaged values and conditional means of heat release rate and species critical to ignition, and by performing a displacement speed analysis and the local combustion mode analysis with an indicator, alpha. Although Tc at 950 K is higher than the autoignition limit, the flame is primarily stabilized by flame propagation rather than autoignition, while at 1100 K, flame stabilization is found to be highly affected by autoignition. As the last part of the dissertation, the characteristics of the flame structure, stabilization, and extinction of counterflow nonpremixed flames of CH4/He versus air at low strain rates are investigated by performing a series of experiments and two-dimensional numerical simulations. It is found from various steady and unsteady 2-D numerical simulations that the dynamics of flame edge plays a critical role in determining the flame stabilization and extinction, and the edge flame is stabilized at a location where negative edge flame propagation speed, Se, balances positive local flow velocity, Ue. The transport budget analysis reveals that despite the negative Se by the diffusive loss of heat and radicals, the edge flame can survive by the help of the convective gain of heat and radicals from the trailing diffusion flame. However, as fuel is highly diluted, a slight inward movement of the edge flame induces a large magnitude of negative Se compared to positive Ue such that the counterflow flame is totally extinguished by the shrinkage of the outer edge flame toward the flame center.ope